This invention relates to the withdrawal of agglomerated solids from a fluid bed of finely divided solids. In particular, this process relates to a coal gasification reactor wherein agglomerated coal ash is withdrawn from a fluid reaction bed of finely divided coal without removal of the finely divided coal particles from the fluid reaction bed.
One of the main sources of atmospheric pollutants today are derived from the combustion of coal in coal fired electric utility boilers. In these installations, a clean fossil fuel such as natural gs is gas a practical substitute for coal in the generation of electricity because of its current scarcity and relatively high cost. Furthermore, the available supply of clean fuel combats pollution more effectively when it is used to provide residential and small commercial needs.
By way of example, the combustion products of coal contribute one-eighth of the total atmospheric pollutants emitted in the United States including one-half of the sulphur oxides and one-fourth of both the nitrogen oxides and particulate matter. Sulphur emissions from coal combustion may be reduced by several methods. These methods include using low sulphur coal; cleaning high sulphur coal by physical methods to remove the sulphur from the coal; removing sulphur from the coal during the combustion thereof; producing a de-ashed low sulphur solid fuel by the solvent processing of coal; and, lastly, gasifying coal and removing the sulphur from the resultant gas prior to combustion of the gasified coal products.
The last method, coal gasification with cleaning of the resultant gas products prior to combustion, appears to offer the greatest reduction in sulphur emissions since most of the sulphur present in the gasified coal appears as hydrogen sulphide. The removal of this hydrogen sulphide, however, from the gasified coal, presents no great problem since several different commercial gas cleaning processes are available today which can reduce the hydrogen sulphide content of a gaseous stream, such as produced in a coal gasification reaction, to less than 10 ppm. In fact, some processes can produce gaseous streams containing hydrogen sulphide of 1 ppm. or less.
A preferred method of gasifying coal is high temperature treatment with air and steam when the coal is finely divided and suspended in a fluid bed. For example, fluid bed reactors for the gasification of coal are illustrated in U.S. Pat. Nos. 2,805,189 and 2,582,712.
In the gasification of coal it is preferred that the gasification of coal it is preferred that the gasification reaction be conducted at high temperatures since this maximizes the production of carbon monoxide and hydrogen which are valuable gaseous fuels. Preferred gasification temperatures are in the range of 1500.degree. to 2000.degree.F and preferably 1600.degree. to 1900.degree.F. Lower temperatures are not desirable since this leads to the production of high amounts of carbon dioxide and water. However, one of the problems encountered in the high temperature gasification of coal is the fusion of ash particles at the high temperatures encountered in the gasification reaction. These high temperatures cause the ash particles to become sticky and agglomerate within the reaction zone. Accordingly, although temperatures in excess of 1700.degree.F are desirable for coal gasification, it is difficult to substantially exceed 1900.degree.F since temperatures in excess of 2000.degree.F lead to the formation of sticky ash particles that can agglomerate to form large ash particles that are difficult to remove from the fluid bed.
One method of removing ash particles from a fluid bed reactor is illustrated in Jequier et al. U.S. Pat. No. 2,906,608, the teachings of which are incorporated by reference herein. In the Jequier apparatus, an inverted conical withdrawal section is positioned in the bottom of the fluid bed reactor. A high velocity air-steam stream is passed up through this inverted conical section and reacts with coal therein to create locally higher temperatures within the confined cone positioned at the bottom of the reactor. Within this inverted cone the ash particles are heated to temperature sufficient to render them sticky whereby they gradually agglomerate and become larger in mass and size. When they reach a predetermined value, size and/or weight, the velocity of the gas stream rising up through the cone becomes insufficient to keep these agglomerated particles in the fluid bed and the particles descend down through the narrow bottom portion of the inverted cone and are withdrawn from the fluid bed reaction zone in an efficient manner. Because the velocity of the gaseous material passing up through the cone always exceeds the settling velocity of the finely divided coal particles in the fluid bed per se, the agglomerated ash particles can be selectively removed without removal of the coal particles from the fluidized bed proper.
A problem associated with the Jequier et al. apparatus is that extremely high temperatures are present in the conical withdrawal section. For example, the temperatures within the conical withdrawal zone are at least 100.degree. and often 200.degree. higher than the temperatures encountered in the fluid bed proper. Since the abrasive agglomerated ash particles are in constant physical contact with the walls of the cone and because of the high temperatures present therein, exotic expensive alloys are required to manufacture a long lasting withdrawal cone.